Photoelectric cell oscillation circuit



27, 1938. H A IAM s @M1389 PHOTELECTRIC CEU, ov LYJAVCN GIRCJT Filed LLC1S?. 1950 WITNESSES! WW' wm@ h ATTORNEY Patented Dec. 27, 1938 UNITEDSTATES PA orric PHOTOELECTRIC CELL OSCILLATION CIRCUIT ApplicationDecember 12, 1930, Serial No. 501,878

1 Claim.

My invention relates to photo-sensitive devices and their uses, moreparticularly to novel circuit arrangements embodying such deviceswhereby new and useful results may be obtained.

f, It is one object of my invention to provide means for increasing thesensitivity of a photocell or other analogous light-sensitive device.

Another object of my invention is to provide photo-cell means havingamplification characlo teristics.

Another object of my invention is to provide an oscillation generatorvembodying a photosensitive device.

Another object of my invention is to provide i5 means for expanding thefield of usefulnes of photo-sensitive devices.

Another object of my invention is to provide light-sensitive meanscapable of selectively generating alternating or direct current.

go A photo-sensitive device, according to the prior art with which I amfamiliar, normally comp-rises an evacuated or a gas-iilled tubecontaining apair ci electrodes, e. g., an anode and a cathode, thecathode being coated or impregnated with a photo-sensitive material. Incertain cases, a third electrode has been added to photo-cells for thepurpose of neutralizing or substantially reducing the capacity eiiectexisting between the anode and the cathode.

"zo According to my invention, I have provided novel means in the formof circuit arrangements embodying photo-sensitive devices whereby suchphoto-sensitive devices may be caused to operate or function to obtainnew and useful results ,5 heretofore never associated with the use ofsuch devices.

Prior uses of photo-sensitive devices in the art have been basedprimarily upon the characteristic of such devices to alter theireliective inter- ,0 nal resistances in response to light waves strikingtheir cathodes and thus to correspondingly alter the currents inelectric circuits of which such devices may be elements. When employedin this relation, the device finds a variety of 15 applications in thearts.

I have found that, in addition to its use as a light-responsiveresistor, a photo-sensitive device may be caused to function as anamplifier and as an oscillator, in a manner somewhat analogous ,n to theoperation of a three-element vacuum tube, at the same time increasingthe sensitivity of the device, as compared with photo-sensitive devicesas normally employed.

In order to employ a photo-sensitive device as ,5 an amplifier oroscillation generator, I provide a grid electrode in addition to theanode and the cathode, the grid, however, to function in a mannerclearly distinguishable from its use in de- Vices of the prior art whichdisclose grids in combination with an anode and cathode fol'neutralizing purposes.

Further description of my invention will be developed in connection withthe accompanying drawing wherein Figure 1 is a circuit diagram, partlyin block form, illustrating one application of my invention wherein aphoto-sensitive device is caused to function as an amplifier andoscillator.

Fig. 2 is a circuit diagram embodying a lightsensitive device accordingto my invention, wherein the amplifying characteristic of the photocellmay be made more pronounced.

Fig. 3 is a diagram representing current changes in a photo-sensitivedevice when employed according to the teachings of my invention.

Fig. 4 is a circuit diagram illustrating an oscillator of the Hartleytype employing a photosensitive device in lieu of a thermionic device.

Fig. 5 is a circuit diagram illustrating a signaling system embodying myinvention.

Referring more particularly to the drawing, the system disclosed in Fig.l is adaptable for use in the reproduction of speech and music from atalking movie film, as illustrated.

A photo-sensitive device l, comprising three electrodes; an anode 3, acathode 5, having a coating or impregnation of photo-sensitive material,and a grid l; is so positioned as to receive a beam of energy from asource 9 directly against its photo-sensitive electrode E. the source isdirected through an optical condensing system comprising a plurality ofcondensing lenses ll and i3 in combination With an opaque shield l5slitted to provide a beam of suilicient dimensions and intensity for thepurpose intended.

A motion-picture film ti, having a sound track thereon, is caused topass between the optical condensing system and the photo-sensitivedevice, the optical system being so adjusted as to concentrate the beamof energy upon the sound track of the film only.

The energy, striking the cathode 5 of the photocell, will, accordingly,be modulated at audio frequencies by reason of, and in accordance with,t

the light and the dark areas on the sound track of the nlm representingspeech or music. By reason of the circuit arrangement disclosed and tobe described, the modulated beam of energy will be transformed back, incorresponding sound The beam from waves, to render audible a duplicationof the original program registered on the film.

Associated with the photo-sensitive device is a grid'circuit comprisingthe grid 1, a source of high potential I9 and the cathode 5, and ananode circuit comprising the anode 3, and an oscillatory circuit 2ladjustably'contacting the source of high potential I9 at an intermediatevalue. The tuned circuit is coupled to the input circuit 23 of a firstdetector to which is also coupled a local oscillator. The output of thefirst detector is fed to an intermediate-frequency amplifier, then to asecond detector and subsequently amplified, through audio-frequencyamplifiers, to be impressed across the terminals of a loud speaker.

Except for the photo-sensitive device I and its associated circuits, thecircuits represented in block form are analogous to those embodied insuper-heterodyne receivers. However, while I have shown thephoto-sensitive device in conjunction with a super-heterodyne amplifier,it

may, with equal effectiveness, be employed inA any suitable amplifyingsystem.

The operation of the system described above may be explained as follows:Concentrating rst of all on the photo-sensitive device and itsassociated grid and plate circuits, attention is directed to the factthat the potential on the grid is of a value materially higher than thaton the anode. Light striking the cathode 5, which is photo-sensitive,will cause an emission of electrons toward the grid l. Because of thehigh speed attained by the electrons, by reason of the high positivepotential on the grid l, a number of the electrons will Vshoot past thegrid and will be attracted, because of positive plate potential, againstthe plate or anode 3, the effect of which will be to knock off from theanode a proportionately'larger number of electrons which may be referredto as a secondary emission.

Under normal circumstances, the electrons emitted from the anode wouldimmediately reenter the electrode, but, by reason of the fact that thepotential on the grid is of a higher positive value than that of theanode, a force of attraction will be exerted on the electrons, with theresult that a flow of electrons will take place from the anode to thegrid, thereby establishing a current,.according to the popular View,from the grid to the anode. 'I'his secondary current may be made manytimes greater in magnitude than the current from the anode to thecathode which is due to the primary emission.

In Fig. 2, I have illustrated circuit connections whereby morepronounced amplifying characteristics may be obtained. The outputcircuit 25 of the photo-sensitive device, as here shown, cornprises thegrid circuit instead of the anode circuit, as described in connectionwith Fig. 1, wherein the amplifying characteristic is the result ofobtaining a secondary emission of elec- `currents will find a returnpath through the grid circuit. Consequently, the output of the devicethrough its grid circuit will be greater than that through either itsanode or its cathode circuit and, accordingly, its amplifyingcharacteristic will be of a greater degree.

In Fig. 3, I have illustrated, by means of a curve, the variations inplate current, as plotted against changes in the plate potential, thegrid potential remaining substantially constant. When the anode voltageis low, few electrons from the cathode reach it and these will betraveling at low velocities only. The number of electrons knocked fromthe plate, therefore, will be smaller than that of the primary electronsarriving at the anode, so that the current in the anode circuit will besmall but of a positive value. This is represented by the small hump inthe curve in the positive direction for low values of plate potential.As the anode potential is increased, the electrons will arrive at theanode in greater numbers and at greater velocities. The secondaryemission will, accordingly, be greater and will increase at a much-morerapid rate than that of the primary electrons reaching the plate. Theresultant plate current will, therefore, change direction and becomenegative. Further increases in plate potential will result in anincrease in current in the negative direction until a maximum value isreached which will occur when the plate potential approaches that of thegrid. 'Ihis point of maximum value is represented at the lowermost bendof the curve. Additional increases in plate potential will cause theplate current to become less and less negative or more positive, sincethe grid, under the above conditions, will be unable to attractelectrons knocked from the plate. The current will ultimately reversedirection and become positive again and will increase in value untilsaturation is reached.

It will thus be apparent that operation along a portion of the curve A-Ewill give a negative resistance effect, which, therefore, satisfies thecondition for the generation of oscillatory current. It will also beapparent that, by fluctuating the plate potential about points A or B,an alternating current may be produced in the output circuit. If it isdesired to produce a direct current in the output circuit, similar tothat of a therrnionic device, the photo-sensitive device may be operatedat such point on the curve Yas is designated by C or D, whereby amodulated light beam, striking the cathode of the device, may befaithfully reproduced in the form of an electric current. This featurewill rind many applications in the art, such as in photo-phone work orthe like, and possesses the advantage over the ordinary photo-cell inthat the output energyrin the plate circuit will be of greater Value,being sumcient, in some cases, to permit of the elimination of a stageof amplification. If the grid circuit is employed as the output circuit,the advantage will be even more pronounced.

Referring back to Fig. l, the photo-sensitive device, in the circuitarrangement illustrated, may be made to oscillate and, when in thisstate, its frequency will be determined by the oscillatory circuitV 2lin the anode circuit of the device. Variations in the current of thecircuit, as caused by the changes produced in the beam of energy whenpassing through the film, will appear as modulation on thehigh-frequency current developed in the device.

The modulated current is heterodyned with the high-frequencyI currentfrom the local oscillator and is detected to an intermediate frequency.The remaining transformations, up to 7 the time the signals arereproduced in the loud speaker, are the same as in any super-heterodynereceiving system.

If necessary, an additional source of light of adjustable intensity maybe provided, whereby to flX the steady-current level of the system.

Another important and useful application of my invention may be made inconnection with facsimile transmission, of the type described in thepatent to Brandon No. 1,687,489. In systems of the type disclosedtherein, a beam of light is caused to traverse a view to be transmitted,the light, as aected by the picture elements, being translated intocorresponding electric current through its effect on a two-electrodephoto-cell. For amplifying and transmission purposes, a choppermechanism, comprising a perforated or slitted disc, is rotated in thepath of the light to cause the beam of light to be interrupted at acomparatively high frequency, which is reproduced in the current of thephotocell circuit.

By employing a photo-cell oscillator of the type disclosed in Fig. 1, inlieu of the photo-Cell circuit disclosed in the patent, it will bepossible to eliminate the chopper mechanism, by generating ahigh-frequency current in the oscillator and modulating it with thepicture-modulated light beam.

In Fig, 4, I have illustrated another application of my inventioncomprising the combination of a three-electrode photo-cell with anoscillatory circuit of the Hartley type, wherein the three-electrodethermionic device is replaced by the photo-sensitive device 33.

The operation of the oscillator will be under the influence or contro-lof a light source 35 which is adapted to throw a beam of light upon thecathode of the device. By causing the light beam to change intensity ina predetermined or desired manner, the oscillator may be made tofunction or stop functioning at will, o-r the oscillatory current may bemodulated according to picture elements or the like being scanned fortelevision, or the current may be modulated according to code signals,in which case, the light-interrupting means will constitute a simplenon-sparking keying means. Under certain conditions, it may be desirableto provide a steady beam of light for maintaining the oscillator inoperation and then employing an independent source of light formodulation purposes.

In Fig. 5, I have illustrated another application of my invention in theform of a signal system embodying a three-electrode light-sensitivedevice 3l. The device, in combination with its locally associatedcircuits, comprises an oscillator of substantially the same type as thatdisclosed in Fig. 1, the frequency of which may be determined by theoscillatory circuit 39. The output circuit is inductively or otherwisecoupled to the grid circuit 4| of a grid-glow tube 43, the anode circuit45 of which may be coupled to a source of potential, such as a -cyclepower source. In the anode circuit to the cathode is provided a relay 41adapted to be actuated during periods of current flow in the circuit. Acurrent-limiting resistor 49 is also provided for protective purposes.The operation of the relay 4l may be made to depend upon predeterminedchanges in the potentials induced from the oscillator which would resultin the glow tube functioning to bring about the necessary changes in theanode current. A system of the type described would be well suited fortrail-lc control work, the light-sensitive device being mounted on oneside of a street near an intersection and a steady beam of light beingdirected against it from the other side at such elevation that a motorcar, approaching the intersection, must intercept the light beam toproduce the required changes for operating the signal light, which wouldbe connected in the relay circuit. If it is desired to 'control traiiicat night only, the light-sensitive device may be so located as to beresponsive to the light beams from the automobile headlights.

From the above description of my invention. it will be apparent that Ihave disclosed means whereby the objects of my invention are attained. Ihave shown how, by employing a grid electrode and utilizing thesecondary emission of a photo-cell of the type described, it is possibleto increase the sensitiveness of the cell and impart amplifyingcharacteristics to it. I have also shown how such cells may be employedas oscillators in much the same manner as thermionic devices, and alsohow, by adjust- :1.v

ing the anode potential on the device, it will be possible to obtain, atwill, either alternating or direct current in the output circuit of thedevice.

Many changes within the scope of my invention may suggest themselves tothose skilled in the art, as well as many applications of photo-cellsfor use in the manner described by me. I do not desire, therefore, to belimited to the details set forth above, except insofar as isnecessitated by the prior art and the appended claim.

I claim as my invention: l

In combination with a photo-sensitive electrode and a source of radiantenergy incident upon said electrode, means for varying the intensity ofsaid incident energy, a second and a third electrode arranged tocooperate with said photo-sensitive electrode, means and circuits forimpressing a potential on said electrode relative to saidphoto-sensitive electrode and a higher potential on said third electroderelative to said photo-sensitive electrode, 'an oscillation networkassociated with one of said circuits, said circuits beingelectro-magnetically coupled, and a load circuit adapted to derive powerfrom one of said circuits.

HARLEY A. IAMS.

